The problem of back scatter as a result of aerodynamic forces acting at a nip between members of a non-impact, electrostatic printing system, is recognized in U.S. Pat. No. 5,499,085 (the disclosure of which is hereby incorporated by reference herein). The system and method of the U.S. Pat. No. 5,499,085 patent are extremely effective in controlling trailing edge dusting on the MIDAX 300 electrostatic printer, as well as other printers. However, there are some other types of electrostatic printers which have configurations that make the particular mechanism of the U.S. Pat. No. 5,499,085 patent for preventing back scatter--namely, applying an electrical bias of the same polarity as toner to the impression cylinder--not as practical. For example, in the Nipson-Printing Systems Varypress, and other types of non-impact electrostatic printing systems having semi-conductive elastomeric rollers, or similar components, or having component geometries which differ significantly from the preferred embodiments of the U.S. Pat. No. 5,499,085 patent, some other mechanism is desirably used in order to prevent back scatter.
According to the present invention an electrostatic printing system, and method of transferring charged poundered toner from an imaged member to a web of imagable material (such as paper), are provided which solve the problem of edge dusting (toner back scatter) in a wide variety of printer component embodiments, including the Nipson-Printing Systems Varypress. According to the present invention it is possible to operate the Nipson-Printing Systems Varypress at high speeds (i.e. over 150 feet per minute, e.g. about 200-300 feet per minute) while achieving excellent print quality, and in a manner--for the particular configurations involved--with greater effectiveness than the mechanism used in the U.S. Pat. No. 5,499,085 patent. According to the present invention the two main mechanisms that may be utilized include a particularly configured electrode biased against the web face which engages the impression cylinder at the nip area, and a shielded corona generator at that area.
According to one aspect of the present invention a non-impact electrostatic printing system is provided comprising the following components: An image member to which poundered toner having a first polarity is applied. A semi-conductive elastomeric roller having an outer periphery, the outer periphery positioned to form a nip with the image member, the roller biased to a second polarity opposite the first polarity. The elastomeric roller rotatable about a first axis (e.g. by a conventional motor of any type). A web of imagable material which passes through the nip to have toner from the image member transferred thereto, the web having a first face which moves into engagement with the elastomeric roller, and a second face which engages the image member at the nip. An idler roller rotatable about a second axis substantially parallel to the first axis, and having an outer periphery spaced from the elastomeric roller outer periphery and for engaging the web first face and for guiding the web to the nip. And means for applying an electrical potential of the first polarity to the web at the first face thereof between the idler and elastomeric rollers, of sufficient intensity so as to substantially prevent back scatter of toner as a result of aerodynamic forces acting between the web and the image member.
In one embodiment the electrical potential applying means may comprise a conductive electrode mechanically biased into contact with the web (e.g. paper) first face, and connected to a power supply having the first polarity (e.g. positive). The conductive electrode may have a tapered first end extending at least partially into the nip so as to maintain the first polarity through the web onto the toner until the web and the image member are in intimate contact. The conductive electrode may be spring loaded, and the spring load actually biases the web into contact with the image member prior to the web actually engaging the elastomeric roller (impression cylinder). The conductive electrode is at all times spaced from the elastomeric roller and the idler roller, however. The poundered toner may have the first polarity imparted thereto, or reinforced therein, by a precharging station disposed prior to the conductive electrode and the nip in the direction of the image member movement. The image member may comprise a belt or other conventional structure, but preferably comprises an image cylinder rotatable about a third axis substantially parallel to the first and second axes.
In another embodiment, the electrical potential applying means comprises a corona generating device connected to a power supply of the first polarity and shielded from the rollers. For example, the corona generating device may comprise a single corona wire (or multiple wires), and the wire may be shielded from the rollers by a substantially arcuate shield. For example, the corona wire and shield may be positioned with respect to the nip so that the web is in substantially intimate contact with the image member before the web is shielded from ions emanating from the corona wire, and the elastomeric roller outer periphery may actually engage the web after the web is in substantial intimate contact with the image member.
Other mechanisms (such as belts, rollers, or other mechanical or electrical or electromechanical components) may be used as the means for applying the electrical potential as long as positioned in operative association with the web at the first face of the web adjacent the nip.
According to another aspect of the present invention a non-impact electrostatic printing system is provided comprising the following components: An image cylinder to which toner having a first polarity is applied and rotatable about a first axis. An impression cylinder having an outer periphery, the outer periphery positioned to form a nip with the image member. The impression cylinder rotatable about a second axis substantially parallel to the first axis. A web of imagable material which passes through the nip to have toner from the image cylinder transferred thereto, the web having a first face which moves into engagement with the impression cylinder, and a second face which engages the image cylinder at the nip. And means for applying an electrical potential of the first polarity to the web at the first face thereof adjacent the nip.
In one preferred embodiment the electrical potential applying means comprises a conductive electrode mechanically biased into contact with the web first face and connected to a power supply having the first polarity, the conductive electrode have a tapered first end extending at least partially into the nip so as to maintain the first polarity through the web onto the toner until the web and the image cylinder are in intimate contact. In another embodiment the electrical potential applying means comprises a corona wire (one or more) connected to a power supply of the first polarity and shielded from the impression cylinder by a substantially arcuate shield.
According to another aspect of the present invention a method of transferring charged toner from an image web to a web of imagable material, such as paper, having first and second faces, using an impression cylinder, and a nip provided between the image member and the compression cylinder, is provided. The method comprises the steps of: a) Providing poundered toner having a positive first polarity on the image member. b) Moving the second face of the web of imagable material into contact with the image member adjacent the nip, at a speed greater than 150 feet per minute, so that toner transfers from the image member to the web second face. And c) applying an electrical potential of the positive first polarity to the web at the first face thereof prior to and adjacent the nip, the potential of sufficient intensity so as to substantially prevent back scatter of poundered toner as a result of aerodynamic forces acting between the web and the image member.
Step c) may be practiced by mechanically biasing an electrically conductive electrode, connected up to a power supply of the first polarity, into contact with the web first face. Step c) may be further practiced so as to maintain said first polarity through the web onto the toner until the web and the image member are in intimate contact, spring loading actually biasing the web into contact with the image member prior to the web actually engaging the impression cylinder.
Alternatively, step c) may be practiced by applying electrical potential using a corona generating device connected to a power supply of the first polarity and shielded from the impression cylinder. Step c) may be further practiced by using as the corona generating device a corona wire shielded from the impression cylinder by a substantially arcuate shield.
It is the primary object of the present invention to effectively prevent back scatter of toner in particular types of electrostatic printing systems. This and other objects of the invention will become clear from an inspection of the detailed description of the invention, and from the appended claims.